Summary
High-salinity water such as seawater, or formation brines, is frequently
injected in carbonate reservoirs. Ion interactions between injection water,
reservoir fluids, and rock surface are quite complex. It has recently come to
be believed that the chemistry of injection water can significantly enhance oil
recovery. Several reaction mechanisms were suggested, including rock
dissolution, change of surface charge, and/or sulfate precipitation.
This study attempts to characterize the electrokinetics of limestone and
dolomite suspensions at 25 and 50°C. In addition, reaction mechanisms at the
water/rock interface were established. Synthetic formation brine, seawater, and
aquifer water were chosen from Middle East reservoirs. Carbonate particles were
soaked in high- and low-salinity water. A phase-analysis-light-scattering
(PALS) technique was used to determine the zeta potential (surface charge) of
carbonate particles over a wide range of pH, ionic strength, and
temperature.
Zeta potential of limestone particles was significantly affected by calcium
ion. Low-salinity water created more negative charges on limestone and dolomite
particles by expanding the thickness of the diffuse double layer. Individual
divalent cations decreased the zeta potential of limestone particles in sodium
chloride solutions, while sulfate ions showed a negligible effect. Limestone
particles in high-salinity water had decreased zeta potential. The solubility
of calcium ions increased as temperature was increased and thus created
additional negative charges. The absence of sulfate in aquifer water strongly
influenced the dolomite surface charge. In summary, surface-charge adjustment
from positive to negative can alter the wettability of carbonate rock from
preferentially oil-wet to water-wet. As a result, residual-oil saturation
should be decreased.
© 2011. Society of Petroleum Engineers
View full textPDF
(
662 KB
)
History
- Original manuscript received:
13 September 2010
- Revised manuscript received:
2 February 2011
- Manuscript approved:
31 March 2011
- Published online:
29 September 2011
- Version of record:
13 October 2011
View Cart
